1. Field of the Invention
This invention relates to a process for salt extraction and a device for executing the process.
2. Description of Prior Art
A process for extracting salt from brine in an open settling or evaporation basin is known from R. B. Richards, Grainer Salt (D. Kaufmann, Sodium Chloride, 1960, Chapter 12). In order to accelerate the evaporation process, the brine is heated to a temperature slightly below the boiling point, typically to 90 to 100.degree. C. In this connection, two different ways of proceeding are known. With the first type, the brine is heated by heating tubes extending inside the basin. With the other type, the brine is continuously moved in a cycle through an external heating unit and returned to the evaporation basin. Such a variant of the Grainer process is described in U.S. Pat. No. 2,660,236.
A further variant of the Grainer process is disclosed in U.S. Pat. No. 2,555,340. Heating is not performed by a heat exchanger, but instead, the brine to be heated is subjected to a direct steam jet.
The Grainer process has several disadvantages. The amount of energy to be supplied is relatively high, in particular since the brine is heated to a temperature slightly below the boiling point. For this reason this process was manly employed in Michigan, where heat in the form of steam was available from sawmills. The effectiveness of the process furthermore strongly depends on the climatic conditions. For economical reasons the process is not used in areas with high temperatures and/or high humidity, since the salt yield does not justify the operating costs. A further disadvantage is that the salt obtained is in the form of platelets and is fine-grained, i.e. with a diameter of less than 2 mm.
A process for extracting salt is furthermore known from French Patent Reference 2,447,218, which permits the formation of larger salt crystals than the known Grainer process, but still provides a higher yield than the salt works operated only by solar radiation. Here, too, an open evaporation basin, which is exposed to solar radiation, is used, but in addition a heated nutrient solution of a preestablished concentration is introduced into a lower layer of the evaporation basin in order to change the solubility in these lower layers.
European Patent Reference 0,009,506 describes a process for salt extraction in a tower of a permeable material, over which seawater is sprayed by means of a sprinkler installation. The evaporated portion is collected, raw salt is added and it is repeatedly sprayed over the tower. This process is expensive and only moderately efficient.
A solar seawater desalination plant is described in German Patent Reference 36 12 188, which constitutes a closed system independent of external environmental influences. Seawater is heated in a heat reservoir and is subsequently sprayed by a spraying device over graduation works. The water evaporates in the graduation works, rises and is cooled again on the walls of the installation and is caught in the form of a salt-free condensate. This installation has the advantage that it only requires sunlight. However, it is not suitable for extracting salt because of the weak salt concentration in the seawater.